The present invention relates to a spheroidal graphite cast iron member and a method of producing it.
Since spheroidal graphite cast iron has excellent mechanical strength and elongation, it is widely used in various applications including automobile parts, machine parts, etc. Specifically, spheroidal graphite cast iron species of FCD 700 and FCD 800 in JIS G5502 are used for parts requiring high mechanical strength, and spheroidal graphite cast iron species of FCD 370 and FCD 400 in JIS G5502 are used for parts requiring large elongation. Further, since important parts of automobiles such as suspension parts are required to have good properties such as tensile strength, elongation, fatigue resistance, impact strength, etc., the spheroidal graphite cast iron constituting such important parts should satisfy the above strength requirements. However, the as-cast surface of the spheroidal graphite cast iron has small unevenness due to contact with mold sand and slag inclusion, and such small unevenness is likely to function as starting points of cracking and failure. Therefore, the spheroidal graphite cast iron having an as-cast surface fails to exhibit its inherent mechanical strength sufficiently.
In such circumstances, the inventors have previously proposed a thin high-strength article of spheroidal graphite cast iron having good mechanical strength (U.S. Pat. No. 4,990,194). Specifically, this thin high-strength article of spheroidal graphite cast iron has graphite particles dispersed in a ferrite matrix containing 10% or less of pearlite, and is characterized in that there are substantially no fine gaps between the graphite particles and the ferrite matrix. Such a thin high-strength article of spheroidal graphite cast iron can be produced by pouring a melt having a spheroidal graphite cast iron composition into a casting mold; removing the casting mold by shake-out after the completion of solidification of the melt, while substantially the entire portion of the resulting cast iron product is still at a temperature of its A.sub.3 transformation point or higher; introducing the cast iron product into a uniform temperature zone of a continuous furnace kept at a temperature of the A.sub.3 transformation point or higher, where the cast iron product is kept for 30 minutes or less to decompose cementite contained in the matrix; and transferring the cast iron product into a cooling zone of the continuous furnace to cool the cast iron product at such a cooling speed as to conduct the ferritization of the matrix.
However, unlike in the case of the thin articles of spheroidal graphite cast iron, spheroidal graphite cast iron articles having relatively large thickness for use in parts which should satisfy higher mechanical strength requirements should retain a pearlite phase to show good mechanical strength and at the same time should exhibit improved bending strength. For this purpose, the heat treatment of ferritizing the spheroidal graphite cast iron entirely or mostly is not satisfactory.